π-Stacks of radical-anionic naphthalenediimides in a metal-organic framework

Radical-ionic metal-organic frameworks (MOFs) have unique optical, magnetic, and electronic properties. These radical ions, forcibly formed by external stimulus-induced redox processes, are structurally unstable and have short radical lifetimes. Here, we report two naphthalenediimide-based (NDI-based) Ca-MOFs: DGIST-6 and DGIST-7. Neutral DGIST-6, which is generated first during solvothermal synthesis, decomposes and is converted into radical-anionic DGIST-7. Cofacial (NDI)2•− and (NDI)22− dimers are effectively stabilized in DGIST-7 by electron delocalization and spin-pairing as well as dimethylammonium counter cations in their pores. Single-crystal x-ray diffractometry was used to visualize redox-associated structural transformations, such as changes in centroid-to-centroid distance. Moreover, the unusual rapid reduction of oxidized DGIST-7 into the radical anion upon infrared irradiation results in effective and reproducible photothermal conversion. This study successfully illustrated the strategic use of in situ prepared cofacial ligand dimers in MOFs that facilitate the stabilization of radical ions.

Metal-organic frameworks (MOFs) are prominent platforms that accommodate NDI •− π-stacks in the solid state (22,23). The structural robustness of a MOF prevents collisional quenching (24,25); furthermore, its porous structure facilitates the inclusion of charge-balancing counter cations or electron donors for stimulus-induced electron transfer (26,27). Therefore, extensive research has been conducted on the development of radical-ionic MOFs (table S1). For example, electron donors such as polyoxometalates, 1,5-/2,6-dinaphthol, 4,N,N-trimethylaniline, and tetrathiofulvalene as guest molecules were incorporated into MOF pores to stabilize the radical-anionic states (28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40). As alternatives, MOFs comprising π-electron-deficient ligands as electron acceptors and π-electron-rich ones as electron donors were prepared (35,36). In addition, cofacially stacked redox-active moieties, such as thiazolo [5,4-d ]thiazoles and viologens, which exhibit through-space intervalence charge transfer phenomena or radical-radical interactions, were used to construct stable radical-ionic MOFs (25,33,(41)(42)(43)(44). However, thermodynamically stable inorganic secondary building units (SBUs) predominantly govern the overall structures of MOFs; consequently, there is a lesser likelihood of obtaining ligand π-stacks (45)(46)(47)(48). Moreover, sustaining a radicalionic state under solvothermal synthesis conditions is known to be challenging (49,50). Therefore, only a few MOFs, which were electrochemically synthesized or included interactions between the NDI units and electron donors, were synthesized in radicalanionic states, and most of the NDI-based MOFs were formed in neutral states (table S2). Consequently, external stimuli such as ultraviolet-visible (UV-vis) light, reductants/oxidants, and electric fields were needed to induce electron transfer and then afford the radical-ionic MOFs (50). However, the generated radical-ionic ligands were not homogeneously distributed throughout the crystals, and the rigidity of the frameworks hindered the structural optimization of the radical-ionic states. These factors have impeded the crystallographic visualization of the radical states by SCXRD; in some cases, the radical formation does not involve notable structural transition. Here, we hypothesized that solvothermal synthesis at a sufficiently high temperature in the absence of an oxidant should facilitate anion/NDI-π interactions that trigger electron transfer from the anion to the NDI to generate NDI •− (51)(52)(53). Subsequently, a MOF composed of NDI radical-anionic dimers (NDI) 2 •− naturally formed through strong π-π interactions is generated. Recently, our group reported the radical-anionic MOF, 150-DGIST-4, formed by the reduction of an NDI-based ligand via anion-π interaction-based electron transfer at high temperatures such as 150°C. However, because the formation of 150-DGIST-4 does not rely on the radical state of the ligand, only some of the ligands were revealed in the radical-anionic states. Many of the MOFs were synthesized at relatively low temperatures, such as 80°t o 120°C, leading to the formation of neutral MOFs (tables S1 and S2). The highly ordered crystallinity of the radical-anionic MOF enables NDI-and NDI •− -state-dependent structural transformations to be observed by SCXRD, leading to an in-depth understanding of associated structure-property relationships (54). Analyzing structural transitions by SCXRD is challenging, owing to radical lability and a lack of established methods for preparing crystalline radical samples (45,55).

Analyzing the NDI •− stability of DGIST-7
Because ligand reduction to (L 2− ) •− and (L 2− ) 2− promotes its formation, DGIST-7 is presumed to be a stable radical. The oxidative quenching of pristine DGIST-7 was examined by electron paramagnetic resonance (EPR) spectroscopy ( Fig. 2A). Samples were prepared by briefly washing the MOF crystals with DMF and then drying under vacuum. The EPR g value of 2.003 indicates the presence of a free organic radical, i.e., (L 2− ) 2 •− (29). Double integration of the EPR peaks revealed that the relative amount of NDI •− grew to 109% over the first 2 hours of air exposure and then gradually decreased to 64% over the next 10 hours and to 3% at 48 hours. Initial peak growth is ascribable to the quenching of EPR-inactive (L 2− ) 2 2− to EPR-active (L 2− ) 2 •− . This quenching is further prolonged in DMF solution; hence, peaks gradually intensified over 3 weeks (Fig. 2B). This unprecedented slow oxidation (stable reduced state) is due to the close proximity of DMA + to (L 2− ) 2 •− and (L 2− ) 2 2− . DMA + is highly disordered and not crystallographically visualizable owing to the coexistence of (L 2− ) 2 •− and (L 2− ) 2 2− in DGIST-7. When DGIST-7 was prepared in the presence of Cu 2+ as the ( •− and a metal cluster were observed by SCXRD (Fig. 3A). In addition, a larger NDI centroid-to-centroid distance was observed (3.309 Å) owing to relatively weak interactions associated with (L 2− ) 2 •− compared with those of (L 2− ) 2 2− . The absence of (L 2− ) 2 2− in DGIST-7-[Cu 2+ ] was further supported by a lack of increase in EPR signal intensity in the initial stage and the observation of comparable peak intensity to the highest observed for DGIST-7 (Fig. 2C). The radical-induced formation of DGIST-7 was further demonstrated by comparing the EPR signal of DGIST-7 with that of 150-DGIST-4 (61). While 150-DGIST-4 also contains NDI •−based ligands, its formation is independent of the radical state of the ligand; consequently, only some of the ligands were reduced to radicals during the solvothermal synthesis of 150-DGIST-4. Accordingly, the EPR signal of DGIST-7 is approximately 161 times more intense than that of 150-DGIST-4 (figs. S13 and S14).

Density functional theory analysis
The dimerization energetics of the NDI moieties in various redox states were examined by density functional theory (DFT) calculations. The ligand has three possible redox states: H 2 L, H 2 L •− , and H 2 L 2− ; each dimerizes with H 2 L to give (H 2 L) 2 , (H 2 L) 2 •− , and (H 2 L) 2 2− , respectively. The dimerization process is energetically favorable in each case, with formation energies of −21.56, −39.05, and −51.37 kcal/mol, respectively (Table 1) (45). Notably, (H 2 L) 2 •− and (H 2 L) 2 2− have lower formation energies than (H 2 L) 2 , which is ascribable to charge delocalization over the cofacially aligned NDI units, as shown in the charge-density maps   4B). In addition, the two added electrons in (H 2 L) 2 2− form a spin-paired singlet state. The energy gains calculated for the formation of (H 2 L) 2 •− and (H 2 L) 2 2− support the notion that H 2 L dimerizes upon reduction, as observed for DGIST-7. However, a positive formation energy (4.68 kcal/mol) was observed when two H 2 L •− molecules dimerize to form (H 2 L) 2 2− due to Coulombic repulsion between negatively charged NDI •− units. (H 2 L) 2 2−•• , which is in a metastable spin triplet state, has an even higher formation energy (14.8 kcal/mol) owing to the lack of spin-pairing.

Generating NDI •− with various stimuli
We assumed that the cofacial arrangement of the NDI dimers in oxi-DGIST-7 is favorable for its reduction to the radical state induced by external stimuli (21). Short, repetitive bursts of infrared (IR) radiation (2 W, 30 s, 30×) reduced vacuum-dried oxi-DGIST-7 to the radical state, which exhibited comparable EPR peak intensities to those of the pristine sample while maintaining crystallinity ( Fig. 2D and fig. S15). Vacuum-dried oxi-DGIST-7 consists of only two DMF molecules per one ligand, as determined by NMR spectroscopy (fig. S10). In general, irradiation with UV-vis light and the presence of electron donors in the pores are required to produce radical states (8,29,68). Radical formation without an additional electron donor suggests that the electron is transferred within the framework. In addition, various common stimuli, such as heat and UV-vis light, also reduced the NDI of oxi-DGIST-7 ( Fig. 2E and figs. S26 to S28). Furthermore, radical-anionic DGIST-7 formed immediately upon exposure to ammonia without any additional stimuli ( fig. S29). Notably, DGIST-6 crystals were only slightly reduced under the same conditions, as confirmed by EPR spectroscopy (figs. S30 and S31).

Photothermal conversion
The NDI bandgap is lowered to the IR region by the addition of electrons, as evidenced by the darkness of the pristine DGIST-7 crystals; this observed IR absorption prompted us to conduct photothermal conversion experiments (69)(70)(71). The temperature readily increased to 150°C within 1 min when pristine crystals were irradiated with a 1-W 808-nm laser (Fig. 5A). Solvent-evacuated oxi-DGIST-7 and Li + @DGIST-7 exhibited similar behavior to that of pristine DGIST-7. The efficient photothermal conversion of oxi-DGIST-7 and Li + @DGIST-7 is attributable to their conversions into radical-anionic states by IR radiation. In addition, reliable photothermal conversions were achieved for at least 5 cycles of 1-hour irradiation because oxi-DGIST-7 and Li + @DGIST-7 were readily reduced by exposure to IR radiation alone; hence, electron donor leaching can be neglected (Fig. 5B) (72). Furthermore, the photothermal conversion temperature is proportional to the laser power (Fig. 5C).
This efficient photothermal conversion can be used as a heat source for organic reactions, which was applied to the Knoevenagel condensation of benzaldehyde with malononitrile in DMF as proof of concept (73). Oxi-DGIST-7 crystals were loaded into a polydimethylsiloxane (PDMS) mold (DGIST-7@PDMS) to separate the MOF crystals from the reactants (Fig. 5D and fig. S32). The average temperature of the solution was approximately 60°C when irradiated with 2-W 808-nm IR light, while the local temperature of DGIST-7@PDMS increased to 219°C (fig. S34). A conversion yield of 92.6% was observed after 1 hour, which is higher than that (48.2%) obtained by heating the vial on a heating block at 60°C. Localized heating at 219°C presumably accelerates the reaction without solvent boiling.

DISCUSSION
In conclusion, we reported DGIST-7, a doubly interpenetrated Cabased MOF. The cofacially oriented NDI dimers in DGIST-7 stabilize NDI •− and NDI 2− by through-space electron delocalization, spin-pairing effects, and the presence of the nearby stable counter cation, DMA + , as revealed by structural analysis and DFT calculations. Notably, oxi-DGIST-7 was promptly reduced to the radical state by various stimuli. Furthermore, the radical state formed by IR irradiation effectively promoted the photothermal conversion; this process was used to heat an organic reaction. The high crystallinity enabled SCXRD to unambiguously show that Li + is octahedrally coordinated to the anionic [Ca 4 (μ 4 -O)(COO) 8 ] 2− cluster after counter cation exchange. We successfully crystallographically visualized the structural transitions that occur during the radical quenching of DGIST-7.

DFT calculations
DFT was used to study the energetics of NDI-moiety dimerization, with hydrogen-passivated dimer configurations (H 2 L) 2 in various redox states used as simplified MOF models (45). The initial atomic structure was adopted from the π-stack of NDIs in pristine DGIST-7, and the carbons in the carboxylate groups of the ligand dimer were fixed to the corresponding atomic positions in pristine DGIST-7. The rest of the ligand dimer was fully relaxed until atomic forces below 0.01 eV/Å were achieved. DFT total energy calculations were performed using the generalized gradient approximation introduced by Perdew et al. (74) and the projector-augmented wave (75) method as implemented in the Vienna Ab initio Simulation Package (VASP) code (76,77). An energy cutoff of 400 eV was used for the plane-wave part of the wave function, and van der Waals interactions were included using the DFT-D2 method (78). The calculated interplanar distances in (H 2 L) 2 , (H 2 L) 2 •− , and (H 2 L) 2 2− are listed in table S4. The added electron in (H 2 L) 2 •− (or a pair of electrons in (H 2 L) 2 2− ) participates in the π-bonding of the ligand dimer. The spin density of the EPR-active (H 2 L) 2 •− is also plotted in fig. S19, which clearly shows π-bonding character between the ligands.

Synthesis of H 2 L
Naphthalene-1,4,5,8-tetracarboxylic acid dianhydride (2680 mg, 10 mmol) and 4-amino-3-hydroxybenzoic acid (3093 mg, 20.2 mmol) were added to DMF (40 ml) in a 250-ml two-neck round-bottom flask. The solution was magnetically stirred and refluxed under a stream of N 2 for 18 hours. After cooling to room temperature, the solution was added dropwise to deionized water (600 ml) and cooled in an ice bath. The obtained precipitate was collected and washed six times (each) with EtOH and acetone and then dried under reduced pressure at 120°C for 12 hours. 1

Preparation of oxi-DGIST-7
After washing three times with fresh DMF, the DGIST-7 crystals were briefly immersed in distilled water and then washed nine times with DMF and three times with EtOH. The resulting crystals were soaked in EtOH for 7 days, with the EtOH refreshed daily.

Preparation of Li + @DGIST-7
After washing three times with fresh DMF, the DGIST-7 crystals were briefly immersed in distilled water and then washed nine times with DMF and three times with EtOH. The resulting crystals were soaked in 3.5 M LiCl·H 2 O in a 1:1 (v/v) mixture of MeOH and EtOH for 13 days, with the Li + solution refreshed daily. Last, Li + @DGIST-7 was washed several times with fresh EtOH to remove residual (weakly adsorbed) Li + . ICP-OES revealed that Li + @DGIST-7 has a Li:Ca molar ratio of 0.515:1.000, which is close to the theoretically expected ratio of 0.5:1.0.

Activating DGIST-7 and Li + @DGIST-7
The pristine DGIST-7 crystals were washed three times with fresh DMF, briefly once with distilled water, and then again several times with fresh DMF. The DGIST-7 and Li + @DGIST-7 crystals were thoroughly washed with EtOH and cyclohexane for freeze-drying. After as much as possible of the cyclohexane had drained, the MOF crystals were frozen at 253 K and then vacuumed at −10°C for 12 hours to remove cyclohexane. The obtained samples were further vacuumed at 80°C for 12 hours.

Radical (NDI •− ) stability testing
The pristine DGIST-7 crystals were washed several times with DMF and dried under vacuum for 18 hours. DGIST-7 crystals (10 mg) were added to a glass tube under inert conditions (Ar). The tube was exposed to ambient atmosphere in the dark, and variations in the amount of NDI •− were monitored by EPR spectroscopy at a power of 1 mW, a modulation width of 0.01 mT, a time constant of 0.03 s, a conversion time of 30 s, a modulation amplitude of one time, and a temperature of 25°C.

Generating NDI •− with various stimuli
The oxi-DGIST-7 crystals were washed several times with DMF and dried under vacuum for 18 hours. The ratio (1:2.01) of H 2 L and DMF in vacuum-dried oxi-DGIST-7 was determined by NMR spectroscopy ( fig. S10). DGIST-7 crystals (10 or 30 mg for ammonia exposure experiments) were added to a glass tube under inert conditions (Ar), after which they were exposed to various stimuli, including heat (150°C), IR, UV, visible light, and ammonia. NDI •− formation was monitored by EPR spectroscopy at a power of 1 mW, a modulation width of 0.01 mT, a time constant of 0.03 s, a conversion time of 30 s, a modulation amplitude of 10 times for figs. S26 to S30 and one time for fig. S31, and a temperature of 25°C.

Fabricating DGIST-7@PDMS
The oxi-DGIST-7 crystals were loaded in a PDMS mold for use as a heat source for an organic reaction without directly interacting the MOF crystals with the reactants. Sylgard 184 silicone elastomer and its curing agent were mixed in a 10:1 weight ratio. Ten drops of the PDMS solution (about 260 mg) were added to a 2-ml vial. A 2-mmdiameter wooden stick was placed at the center of the PDMS solution to create a hole, after which the vial was maintained at room temperature for 6 hours to remove bubbles. In sequence, the vial was then placed in an oven at 60°C for 12 hours to solidify the PDMS mold, and the oxi-DGIST-7 crystals (5 mg) were carefully added into the hole, after which the PDMS solution (four drops, about 100 mg) was added onto the crystals and cured at 60°C for 12 hours. The color of the MOF crystals changed from light yellow to dark brown during curing, indicative of the reduction of oxi-DGIST-7 to DGIST-7.

Knoevenagel condensation
Benzaldehyde (0.102 ml, 1 mmol) and malononitrile (0.063 ml, 1 mmol) were dissolved in DMF (1.5 ml). Condensation was conducted for 1 hour under six heating conditions: (i) without heating and with heating at (ii) 40°, (iii) 50°, and (iv) 60°C using a heating block, and with heating using (v) 1-W and (vi) 2-W IR laser light. The solutions were magnetically stirred during the reaction. The reaction progress was monitored by NMR spectroscopy, with conversion rates calculated from peak integrals ( fig. S35).

Supplementary Materials
This PDF file includes: Supplementary Text Figs. S1 to S35 Tables S1 to S4 References Other Supplementary Material for this manuscript includes the following: Data S1 to S5